Abrasive grinding and sanding machines are finding increased use in many areas where it is necessary to perform surface operations on workpieces. Machines of this type generally consist of one or more heads, each of which includes an endless abrasive belt moved at relatively high speed around driving and driven rollers. The abrasive belts and rollers may be relatively wide (i.e., on the order of several feet in width), and thus capable of surfacing workpieces of substantial width, such as plywood panels. These machines have many advantages over conventional surfacing apparatus, among them time and cost efficiency, better accuracy and safer operation.
One additional primary advantage is that two opposed heads may be provided for the abrasive grinding or sanding machine, which permits both surfaces of a workpiece to be processed simultaneously. Dual head wide belt sanding machines are now commonly used for surfacing and dimensional control for wood panels such as plywood. Spacing of the dual heads may be controlled to the closest thousandth of an inch, thus permitting extremely large panels to be quickly and efficiencly surfaced with great accuracy. Abrasive sanding machines have also been used to a limited extent for dimension lumber, which is generally rough cut from softwood, and also for hardwood planks of various sizes. However, various problems have arisen because lumber of this type often is warped significantly, and also because it may vary significantly in thickness in rough cut form.
In dual head abrasive sanding machines, it is imperative that each workpiece be uniformly surfaced on both sides. Otherwise, the workpiece will be improperly surfaced, or not surfaced at all in some areas. In many cases, the resulting defective workpiece must be discarded. For rough cut dimension lumber that is warped in one way or another, or which varies in thickness in rough cut form, it is most difficult for the machine to accomplish its intended function even though the sanding heads are spaced apart a predetermined amount and rigidly held in this position.
For example, a length of dimension lumber which has a torsional warp about its longitudinal axis cannot pass between the sanding heads in a manner that causes each side to be uniformly surfaced. The warpage will cause excessive surfacing on one side and insufficient surfacing on the other in the areas of maximum warpage.
Some prior art machines have used rigid or fixed position rollers in feed mechanisms to force the warped lumber through the sanding heads in a manner which causes uniform surfacing. However, even though the length of lumber may be temporarily held in a nonwarped position as it passes through the sanding heads, the best possible end result is a warped length of lumber having both sides surfaced. If the warp is particularly bad and there is no resiliency on the part of the feed system, it is often the case that the lumber will become cracked or split as it is forced to comply with the straight line form of the fixed roller feed system. In cases such as this, the lumber must be discarded.
This problem is compounded by workpieces that vary in thickness. In rigid feed systems, once the machine is adjusted to workpieces of a particular nominal thickness, a thinner piece may slip within the feed apparatus, or be guided and surfaced poorly, and thicker workpieces may be too large to enter the machine.